Demodulator



July 11, 1939. H. PITSCH 2,165,764

DEMODULATOR Filed Aug. 4, 1957 OUTPUT MUM/M 7'50 WAVE E/VERGY INVENTOR f/ELM Z/mcw BY ATTOR N EY Patented July 11, 1939 UNITED STATES PATENT OFFICE DEMODULATOR Helmut Pitsch, Berlin,

Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphie, m. b. H., Berlin, Germany, a corporation of Germany 1 Claim.

The demodulation of a carrier with a side band by means of a linear rectifier entails distortions which make themselves felt in the form of combination notes and upper harmonics. This has its cause in the fact that the envelope of the modulated carrier frequency has no sinusoidal form despite the sinusoidal modulation. The application of a rectifier characteristic of parabolic form offers advantages only when the side frequencies control only a very small part of the characteristics, and if the carrier frequency voltage is higher than the voltages of the side frequencies.

In accordance with the invention a demodulation method for single side band telephony is given which operates without distortions and has a wide range of control. The invention resides in that the working point of the grid to which the signalling oscillations are applied is so placed that these oscillations control only a straight portion of the tube characteristic, and that only the carrier frequency is applied to a second grid with whose voltage variation the steepness of the straight tube characteristic varies.

The invention can be practiced on a double rid tube, but more favorable results are obtained when the invention is applied to a tube in which between the two grids a positively biased screen grid exists. The filtering out of the carrier frequency can best be carried out by means of a quartz crystal. Therefore, it will be of advantage to employ the invention in the intermediate frequency part of the receiver, since in this case the filtering means for the carrier frequency need not be varied.

An example of a circuit for practicingthe invention is shown in the figure.

The intermediate frequency carrier together with the one side band are applied to the band filter B and then to the first grid 2 of the tube. The intermediate frequency carrier is furthermore filtered out by means of the quartz crystal Q and applied to the third grid 6. The first grid 2 receives a negative biasing potential across the oscillatory circuit and the third grid 6 receives such a potential across the resistor W. The interposed screen grid 4 which is positively biased has the effect that a voltage variation at the third grid 6 tilts about a fixed point the grid voltage-plate current characteristic appertaining to the first grid 2. The last screen grid 8 has the purpose of increasing the amplification.

The arrangement acts such that the carrier frequency modulates the side band frequencies, thereby producing as in case of any m d lation cies cannot modulate each other, and combina- 1 tion notes can likewise not be produced by the said side band frequencies.

It should furthermore be pointed out that the use of the invention offers the above mentioned advantages only in the single side band teleph- 9 ony, since in the telephony with two side bands a distortion free demodulation can already be assured by means of linear rectification, the in vention therefore relating only to the single side band telephony. It is obvious that the invention can also be used when for the demodulation of a carrier with two side bands only the one side band is employed.

The invention is not confined to the measure of filtering out the carrier frequency, but the carrier frequency may also be produced locally, and then added, for which purpose the demodulation tube may for instance be designed as self oscillating tube such as is customary in the mixer tubes in superheterodyne receivers. The amplification achieved is the higher the larger the amplitude of the carrier frequency is chosen.

I claim:

In a superheterodyne receiver, a demodulator of an intermediate frequency carrier which is modulated by only a single side band frequency, comprising a vacuum tube provided with at least a cathode, a signal control grid, a second grid and an anode, a band-pass filter of intermediate frequency connected between the signal grid and the cathode, a quartz crystal connected between the filter and the second grid for impressing only the intermediate frequency carrier on said second grid, means for applying a negative biasing potential to the control grid for maintaining the operating point of the tube on a linear portion of the grid voltage-plate current characteristic, and means for applying a negative biasing potential to the second grid such that the voltage variations applied thereto will vary the steepness of said tube characteristic about the operating point maintained by the potential on the first grid. 

